May 17, 2015

Excrementary, my dear Watson

Where does sand come from? A simple question, not always easily answered, and certainly not in some isolated tropical island environments. The Maldives Archipelago consists of 22 coral atolls, each containing a multitude of individual reefs, many of which surround a small island rimmed with glistening white sand. Those islands are vital, not only for the inhabitants and the economy, but for biodiversity and the health of the Maldives ecosystems. But they are vulnerable landforms, consisting simply of piles of unconsolidated sand, rarely more than 3m above sea level – and that sea level is rising. Understanding how the islands are maintained, the sources and movement of the sand that builds them, is an important, and, until now, poorly understood process. However, recent work by a team from the University of Exeter, in the UK, and collaborators from New Zealand and Australia, has revealed the details of reef island sustenance – and it all comes down to huge quantities of parrot fish excrement.

The study focused on the small island of Vakkaru (shown above), partly cultivated, partly vegetated, and completely surrounded by its white beaches, lagoon and reef. There is no source of sand other than that reef, for the Maldives are, after all, in the middle of the Indian Ocean and the nearest rivers flowing down to the shore, hills being weathered and eroded, are a very long way away. The whole sedimentary system is entirely biogenic, being run by the teeming ecosystem of the atolls. Exeter’s Chris Perry and his colleagues have meticulously quantified the major sediment-generating habitats, the abundance of different sediment-producing critters in each, and the rates of production. Vakkaru and its reef are little more than a maximum of a kilometre across, and the area of the island itself is less than 0.2 square kilometres, yet every year nearly 700 tons of new sediment is produced. Perhaps 10% of this comes from the broken-up calcareous segments that halimeda, a genus of macroalgae (or seaweed), produces as part of its structure, but more than 85% of the sand generated is comprised of parrotfish faeces.

A variety of parrotfish, particularly the excavator species, Chlorurus sordidus and Chlorurus strongylocephalus, and the scraper species Scarus niger, Scarus frenatus, and Scarus rubroviolaceus, chew up coral in order to extract nutrition from algae, and then excrete the indigestible stuff – as sand-sized grains of calcium carbonate. And significant populations of parrotfish do so in prodigious quantities. It is reported that the native Hawaiian name for the female redlip parrotfish translates to “loose bowels,” and this video is a striking illustration:

All of this sediment manufacture takes place around the reef itself. Some sediment gets flushed out into the deeper ocean, but much is transported, particularly during the monsoon season, into and across the lagoon and up onto the island. During this process, many of the halimeda fragments are further broken up and the dominant sand of the island is parrotfish poop. This illustration from the paper published last month in Geology summarises how the system works:

Many varieties of parrotfish are endangered, but it is clear that their role extends beyond key participants in biodiversity. As the report concludes:

While the need to protect parrotfish populations is commonly based on the need to sustain benthic ecological interactions, this study demonstrates their further critical beneficial role as producers of carbonate sediment and thus as key biogeoengineering species that can sustain local landform maintenance.

It has long been known that parrotfish manufacture sand – although rarely featured in tourism brochures, fish excrement is responsible for many of Hawaii’s gleaming white beaches – but this fascinating new analysis reveals the scale on which these bioengineers work.

[Parrotfish image by Chris Perry from the Science News and University of Exeter reports. Movie by Matthew Duncan. Paper: Linking reef ecology to island building: Parrotfish identified as major producers of island-building sediment in the Maldives, C.T. Perry, P.S. Kench, M.J. O'Leary, K.M. Morgan and F. Januchowski-Hartley, Geology, first published online April 27, 2015]